Two angles are supplementary when they add up to exactly 180°. Together they form a straight line.
A linear pair — two angles sitting side by side on a straight line — is always supplementary. If one angle is 120°, the other must be 60°.
Remember
Supplementary = 180°. Straight line = 180°. The two go hand in hand.
Real World
When you open a door, the door and the wall on either side form two angles that add up to 180° — supplementary angles right there in your house.
Feeling ready?
Put this concept to work with a practice workout.